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Advantages, Disadvantages, and Applications of Three Homogenization Methods: High-Pressure Homogenization, High-Shear Emulsification, and Microfluidization
This article compares three homogenization methods—high-pressure homogenization, high-shear emulsification, and microfluidization—using yam products as an example, aiming to identify suitable solutions for achieving uniform emulsification, extending product shelf life, and ensuring good dispersion.
01
The Origin of Emulsification and Homogenization
In industrial applications, when two immiscible liquids or solid-liquid mixtures are involved, emulsifiers are typically used to achieve oil-water emulsification and homogeneous dispersion. After oil and water phases mix to form water-in-oil or oil-in-water emulsions, they cannot remain stable without suitable emulsifiers to adjust the surface tension of the system. Simultaneously, intense cutting and dispersion are required to break the medium into fine particles, ultimately forming a stable and uniform dispersion system with effective emulsification.
Currently, the application of emulsifiers is not limited to emulsification alone. With their intense shearing effect, emulsifiers can crush powder particles through impact and collision, refining them to a fine particle size. This allows solid particles to fully integrate into the liquid, forming a relatively stable suspension. When the fineness of the material reaches the micrometer or even nanometer level, the system can be considered homogenized.
High-pressure homogenization can refine materials into tiny particles, linking emulsification and homogenization to achieve stable emulsions. Therefore, emulsifiers can also be referred to as homogenizing emulsifiers.
02
Principles of Homogenizers
The primary forces in homogenizers are shear force and pressure. During homogenization, laminar flow effects cause dispersed phase particles or droplets to be sheared and stretched until they break. Turbulent flow effects lead to random deformation of particles or droplets under pressure fluctuations. Cavitation effects occur when high pressure causes small bubbles to burst rapidly, releasing energy and generating local hydraulic shocks and vibrations. Under the combined action of these forces, the material achieves a well-distributed and homogeneous state.
1. Principle of High-Pressure Homogenizers
A plunger pump continuously performs reciprocating motion to draw material into the valve assembly (Figure 1), with the plunger adjustable to control pressure. When the material flows through narrow gaps under high pressure, droplets are first stretched and then broken apart by the turbulent flow as they pass through the valve body. The high-speed liquid jet exiting the valve gap impacts a collision ring, producing intense impact forces. Simultaneously, the pressure drops rapidly and significantly, generating substantial explosive force and instant cavitation. The intense energy release and high-frequency vibrations break down particles or droplets, achieving homogenization, fragmentation, and emulsification of liquid samples. Droplets carry extremely high static pressure energy at the material inlet, which is converted into kinetic energy during homogenization, leading to droplet rupture.
II. Principle of High-Shear Emulsifiers
The shear head consists of a rotor and a stator, which mesh with each other, with each stage of the stator and rotor containing multiple layers of tooth rings. The high-speed rotation of the rotor generates strong centrifugal force, creating a high negative pressure zone that draws the material into the working chamber. Within the gap between the stator and rotor, the material is subjected to comprehensive effects such as shear, centrifugal extrusion, impact tearing, and turbulence, resulting in tension that splits the liquid droplets. After the liquid exits the small holes of the stator, the pressure recovers, leading to cavitation effects. The homogenization head rotates at high speed, shearing, dispersing, and impacting the material. This process refines the material further, achieving homogenization.
III. Principle of Microfluidization Homogenizers
Microfluidization homogenizers primarily consist of a dispersion unit and a pressure-intensifying mechanism. Under the action of the pressure-intensifying mechanism, high pressure generated by a hydraulic pump forces the fluid through a valve core with extremely small apertures, creating fluid streams at several times the speed of sound. These streams rapidly pass through the narrow gaps of the dispersion unit, undergoing intense high-speed collisions. During the collision process, the fluid instantaneously converts most of its energy, and the sharp drop in pressure within the fluid forms supersonic streams. The particles within the fluid collide, cavitate, and generate turbulence, with shear forces acting on nanoscale molecules, ensuring the fluid's components exist in a completely homogeneous state (Figure 2).
III. Comparison of Homogenizers
High-pressure homogenizers primarily rely on the high pressure generated by the pressure system to compress, stretch, impact, and break down materials, with key mechanisms being cavitation and turbulence effects. The design of the homogenizing valve in these machines features relatively large gaps and operates at lower homogenizing pressures, making them susceptible to damage when processing high-hardness particles and difficult to repair. Their advantage lies in their relatively lower cost. High-pressure homogenizers are more suitable for processing soft or semi-soft granular materials.
High-shear emulsifiers mainly utilize the high shear forces generated by the high-speed relative motion between the stator and rotor to shear, tear, and mix materials. Additionally, their strong cavitation effects disperse, refine, and homogenize material particles. Their advantages include large processing capacity, stable product output, minimal disruption to emulsion layers, durability, ease of maintenance, and a wider variety of homogenizing methods. Their drawback is their relatively large size.
Microfluidization homogenizers utilize micron-sized channels to form supersonic jets, which collide intensely with each other, generating extremely strong shear forces. This results in higher homogenizing pressure and finer particle size distribution. However, they are designed for high pressure, have relatively low flow rates, and are comparatively more expensive. A detailed comparison is provided in Table 1.
04
Application Analysis
High-pressure homogenizers, due to pressure limitations, are suitable for processing soft and semi-soft granular materials but are not ideal for highly viscous materials. Ju Jian et al. used an optimized ultra-high-pressure assisted extraction method, which significantly increased the total phenol extraction rate from yam peel, with short extraction time and stable process conditions. He Yongchao et al. processed Chinese yam using high-pressure homogenization and found it could reduce the internal order of starch, decrease anti-enzymatic hydrolysis capacity, and improve starch digestibility, with this trend becoming more obvious as pressure increased. High-pressure homogenizers significantly refine fat globules in various dairy beverages. When used in the production of products like ice cream, they improve the fineness and porosity of the mixture, prevent or reduce liquid separation, and result in a mellower taste.
High-shear emulsifiers are more suitable for processing materials with more fibers or harder particles, allowing mixing, sterilization, and homogenization to be completed simultaneously. Liu Junmei et al. used high-shear emulsification technology to treat soy protein, greatly reducing protein particle size and exposing hydrophobic groups, which is beneficial for forming a gel network and can significantly improve the water-holding capacity of soy protein isolate gels. Guo Weijing et al. treated corn gluten meal using high-speed shear emulsification technology to alter its physical properties, significantly reducing particle size, improving rheology and solubility, increasing starch degradation rate and protein hydrolysis degree, and effectively inhibiting the layering phenomenon of fruit and vegetable juices. High-shear emulsifiers are widely used in products with higher viscosity and emulsification requirements, such as jam and fruit tea.
Microfluidizers can extremely refine the particles of the medium (with an average liquid-liquid homogenization particle size below 1 μm). The homogenized products also gain advantages such as non-settling, high colloidal properties, and high stability, greatly improving the appearance of the final product. They are suitable for industries like new nanomaterials, pharmaceuticals, biotechnology, cosmetics, and high-end beverages. Guo Xiaojun et al. processed yam juice using ultra-high pressure microfluidization technology and found it significantly improved the physical stability of the juice, with no significant change in soluble solid content; brightness significantly increased, the average particle size of particles in the juice significantly decreased, and turbidity and non-enzymatic browning gradually decreased.
Liu Mengpei et al. found that ultra-high pressure microfluidization treatment could better preserve the nutritional components in yam juice. At 80 MPa pressure, amino acids increased, while reducing sugars, total acids, and flavonoids showed a slight decrease. Wang Xiaoyuan et al. showed that high-pressure microfluidization had a good sterilization effect on microorganisms in iron stick yam juice. Microfluidizers can form nanoemulsions and, besides sterilization, can also rupture cell walls to extract useful nutritional components. They can reduce the dimensional size of cellulose to within 100 nm, forming nanofibrillated cellulose.
Human absorption depends on the action of enzymes. The smaller the particle size of a substance entering the human body, the larger the surface area in contact with enzymes for reaction, and the higher the absorption efficiency. Efficiently rupturing cell walls to extract their contents is of great significance for improving human absorption rates. Therefore, selecting appropriate homogenization equipment becomes key in the production process.
Each type of equipment has its advantages and disadvantages. Using a high-pressure homogenizer alone may not achieve good dispersion and grinding effects due to its limited pressure. Using a microfluidizer alone has a relatively small flow rate, while high-shear emulsifiers have intense stirring effects. Therefore, employing a combination of two or three types of equipment to process materials may achieve better homogenization results.
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